of Lysigenous Cavity -formation. 417 
by De Vries 1 in a flower-stalk of Pelargonium zonale. Among 
the flower-buds on this stalk appeared a vegetative bud. 
The flower-buds were removed and the vegetative bud allowed 
to develop, resulting in the conversion of the usually short- 
lived flower-stalk into a permanent vegetative axis. 
If any doubt remains as to the correctness of the view that 
the prevention of the destructive tension is not alone sufficient 
to account for the greatly prolonged vitality of the pith, that 
doubt can be removed by the following results of experi- 
ments : — In Helianthiis tuber osus, Sambucus nigra , Forsythia 
viridissima , Pterocarya fraxinifolia , and Juglans nigra , the 
pith dies, and in the three last collapses also, from several 
to many weeks after secondary growth begins. In all of 
these plants there is a firm zone of xylem of secondary 
formation enveloping the pith before there is any indication 
of dying cells. Casts were placed about the stems of several 
individuals of each of these species after primary growth had 
ended. The cast could thus have no effect in lessening the 
tension between pith and more peripheral tissues, unless in 
secondary growth there were a displacement of the vascular 
ring toward the centre. But such displacement did not occur ; 
the zone of mechanical tissue formed before the application 
of the casts was sufficiently strong to prevent it. In this 
series of experiments the stems that grew well — those that 
increased their diameter greatly in excess of the portion 
held within the cast — preserved their pith alive within the 
cast for a long period, while similar stems that made but 
small growth subsequently to the application of the casts 
preserved the vitality of the pith for a shorter period. 
Whether in these cases of flourishing stems the pith would 
live on indefinitely, as seems to be indicated for Dahlia and 
Vicia Faba already cited, was not determined in the experi- 
ments. But several individuals of Helianthus tuberosus showed 
a living pith within the cast at least six weeks after it had 
died outside the limits of the cast. Similarly, Sambucus nigra 
1 De Vries, Ueber abnormale Entstehung secundarer Gevvebe. Jahrb. f. wiss. 
Bot. Bd. -22 (1891). 
